= Hidden turning-ray zone from nonmonotone spherical slowness
Where $\eta=r/v$ decreases outwards, it increases as a surface ray travels inward, so no surface ray turns inside that interval. Penetrating rays may traverse it and turn deeper. The radius is then not a single-valued function of $\eta$, and angular ray data constrain combined branch contributions rather than the single function inverted by <Herglotz–Wiechert inversion>. Additional structural or observational information is needed to locate the hidden profile.
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